Final answer:
The layer of soil with the most decomposing or recently decomposed vegetative material is the O layer. The soil profile consists of various layers, with the A horizon being the topsoil and the B horizon as the subsoil. Hence, the correct answer is option C.
Step-by-step explanation:
The layer of soil with the most decomposing or recently decomposed vegetative material is the O layer. The O horizon has freshly decomposing organic matter, humus, at its surface, with decomposed vegetation at its base. This layer is crucial as it enriches the soil with nutrients essential for plant growth.
The A horizon is typically considered the topsoil, and the B horizon is known as the subsoil. The thickness of these layers can vary based on environmental factors and the maturity of the soil. For instance, grassland soils tend to have much thicker A horizons with high organic content, making them more agriculturally productive.